Photosensitive materials used in color proofing are of the positive- or negative-working types. A positive-working process is described in U.S. Pat. No. 3,649,268 to Chu and Cohen. Positive-working materials may contain tacky, photohardenable compositions which, when exposed imagewise to actinic radiation, harden in the exposed image areas, resulting in tacky and complementary non-tacky image areas. The image can be developed by applying colored particulate materials such as toners or pigments which selectively adhere to the unexposed tacky image areas.
A negative-working process is described in U.S. Pat. No. 4,174,216 to Cohen and Fan, which teaches a negative-working element having a support; a tacky, non-photosensitive contiguous layer; a photohardenable photoadherent layer; and a strippable coversheet. After, imagewise exposure to actinic radiation, the coversheet is peeled away, carrying with it the exposed areas of the photoadherent layer, and bearing the tacky contiguous layer beneath. These areas of the tacky contiguous layer lying underneath the exposed image areas may then be toned with a finely divided particulate material. Different colored layers can be prepared and assembled in register over one another to form multilayer color proofs, as is well known to those skilled in the art. U.S. Pat. No. 4,053,313 to Fan describes a similar negative-working system which is developed by solvent washout.
Alternatively, a photosensitive imaging system may be precolored with dyes and/or pigments or other coloring materials, developed by washout, contain photoplasticizing agents instead of photohardening agents, and other variations known to those skilled in the art. Negative working systems may be achieved in a number of different ways, for example by utilizing a photosensitive layer which becomes tacky and tonable on exposure to actinic radiation. It is understood that in addition to various photopolymer elements, other non-silver halide systems, e.g., diazo systems, are useful in preparing multicolor proofs. In addition the described protective polymer layers of this invention may be beneficially applied to many types of image bearing surfaces, e.g., a conventional silver halide photograph, where it is desired to provide a clear protective covering having a predetermined level of gloss. It is understood that the polymeric layers of the invention may also be applied to nonimaged glossy surfaces to obtain a desired level of lowered gloss.
Prepress color proofs such as those described above are usually covered with one of four types of protective coverings or coversheets. The first of these is a positive-working photosensitive element as described in Cohen and Fan, above, comprising a photosensitive layer and a polyester coversheet. This is exposed overall to actinic radiation and the polyester coversheet removed. This results in a proof with a desirable 20.degree. gloss level of about 65 gloss units. It suffers, however, from poor durability and is susceptible to crazing, scratching and marring.
The second type of topcoat or coversheet is negative-working photosensitive element as described in Cohen and Fan, above, which comprises a polyester cover sheet, a photosensitive layer, and an organic contiguous layer. This is laminated to the proof and exposed overall with the polyester coversheet left in place. The result is a proof with a more durable finish but the gloss associated with the reflective, highly transparent integral polyester coversheet is unacceptably high, i.e., about 107 gloss units measured at 20.degree.. The unacceptability of high gloss has long been recognized, and considerable effort has been focussed on ways to achieve a durable lower gloss protective coating that is resistant to crazing, scratching marring, etc.
The third type of protective coating or coversheet is an integral layer of plastic film, such as polyester, coated with a non-photoactive adhesive, and applied to the proof by lamination. The net result is indistinguishable from the second type of cover sheet above in that it provides a durable coating, but one which exhibits an unacceptably high gloss.
The fourth type of protective topcoat or covering is described in Taylor, U.S. Pat. No. 5,019,536, which comprises a thin substantially transparent, non-photosensitve, integral polymeric layer on a strippable substrate. The polymeric layer comprises a mixture of slightly incompatible polymers. In one embodiment, a wetting layer is provided on the side of the polymeric layer opposite the strippable substrate, and a covering film may be provided over the wetting layer. This film provides the required gloss, however, having two slightly incompatible polymers increases the possibility for variability in coating solutions and therefore variability in the final product. Also, in the embodiment having the wetting layer and the covering film, the product has exhibited unacceptable adhesion, wherein when the covering film is peeled off, part of the wetting layer and polymeric layer are peeled off with it. Further it is incompatible to flammable solvents which poses some environmental concerns.
A need exists for a photosensitive element comprised of layers which have the required low peel force and the required adhesion balance so the element can be developed using peel-apart development.